A new technique, combining the perfusion of ventral roots with isotonic sucrose solution and the use of spike triggered averaging (see Soc. For Neurosci Abst. 4: 568, 1978), will be applied to a study of the geographical factors influencing the projections of Ia and group II fibers to motoneurons. The exact entry point of each afferent fiber used for triggering will be correlated with the position of the responding pools of motoneurons and the amplitudes of the PSPPs evoked. A new "off-line" averaging technique for studying individual EPSPs elicited by impulses in Ia and group II fibers will be used to compare the synaptic effects of impulses in 9-10 different fibers on the same motoneuron. Anatomical studies on motoneurons labelled with HRP will be continued. During the past year, the HRP technique yielded inconsistent results, but it has been developed into a reliable method. The next step is the study of labelled cells with electron microscopy.